Conductive vias are an important feature of circuit boards. There are two types of vias--filled vias and plated through-hole vias. Filled vias are typically used in co-fired, multilayer ceramic substrates. Plated vias are generally used in single or multilayer epoxy-glass circuit boards, although they can be used with ceramic substrates. Filled vias are solid columns extending between the two surfaces. Plated vias are hollow. This invention relates to plated vias.
Plated vias are used for either electrical or thermal conduction. Plated vias for electrical conduction provide electrical connection between conductors on one side of the board to those on the other, and to inner layer conductors in the case of multilayer boards. Plated vias for thermal conduction transfer heat generated from components on one side of the circuit board to the other for more efficient heat dissipation.
Current processing steps for fabricating plated vias are: (1) forming the via, (2) making the via inner wall surface conductive, and (3) electroplating copper on the circuit board, including vias. Because the via dimension is substantially smaller than that of the circuit board, the copper plating within the via is difficult to control and the thickness distribution tends to be non-uniform along the via. The non-uniformity of copper thickness distribution within the vias directly deteriorates the performance of both electrical and thermal vias. Accordingly, there is a need for an improved process for making vias of enhanced conductivity and uniformity.